Generally, MIMO (multi-input multi-output) is a method that uses a plurality of transmitting antennas and a plurality of receiving antennas. And, this method may be able to improve efficiency in transceiving data. In particular, a transmitting or receiving stage of a wireless communication system uses a plurality of antennas to increase capacity or enhance performance. In the following description, the MIMO may be called multiple antennas.
The MIMO technique does not depend on a single antenna path to receive one whole message. Instead, the MIMO technique completes data by putting fragments received via several antennas together. If the MIMO technique is adopted, a data transmission rate within a cell area having a specific size may be improved or a system coverage may be increased by securing a specific data transmission rate. Moreover, this technique may be widely applicable to a mobile communication terminal, a relay and the like. According to the MIMO technique, it may be able to overcome the transmission size limit of the related art mobile communication which used to use a single data.
FIG. 1 is a diagram for a configuration of a general MIMO communication system. NT transmitting antennas are provided to a transmitting stage, while NR receiving antennas are provided to a receiving stage. In case that each of the transmitting and receiving stages uses a plurality of antennas, theoretical channel transmission capacity is increased more than that of a case that either the transmitting stage or the receiving stage uses a plurality of antennas. The increase of the channel transmission capacity is in proportion to the number of antennas. Hence, a transmission rate is enhanced and frequency efficiency can be raised. Assuming that a maximum transmission rate in case of using a single antenna is set to R0, the transmission rate in case of using multiple antennas may be theoretically raised by a result from multiplying the maximum transmission rate R0 by a rate increasing rate Ri, as shown in Formula 1. In this case, Ri is a smaller one of NT and NR.Ri=min(NT,NR)  [Formula 1]
For instance, in an MIMO communication system, which uses 4 transmitting antennas and 4 receiving antennas, it may be able to obtain a transmission rate 4 times higher than that of a single antenna system. After this theoretical capacity increase of the MIMO system has been proved in the middle of 90's, many ongoing efforts are made to various techniques to substantially improve a data transmission rate. And, theses techniques are already adopted in part as standards for the 3G mobile communications and various wireless communications such as a next generation wireless LAN and the like.
The trends for the MIMO relevant studies are explained as follows. First of all, many ongoing efforts are made in various aspects to develop and research information theory study relevant to MIMO communication capacity calculations and the like in various channel configurations and multiple access environments, radio channel measurement and model derivation study for MIMO systems, spatiotemporal signal processing technique study for transmission reliability enhancement and transmission rate improvement and the like.
In order to explain a communicating method in an MIMO system in detail, mathematical modeling can be represented as follows. Referring to FIG. 1, assume that NT transmitting antennas and NR receiving antennas exist. First of all, regarding a transmission signal, if there are NT transmitting antennas, NT maximum transmittable informations exist. Hence, the transmission information may be represented by the vector shown in Formula 2.s=[s1,s2, . . . ,sNT]T  [Formula 2]
Meanwhile, transmission powers can be set different from each other for transmission informations s1, s2, . . . , sNT, respectively. If the transmission powers are set to P1, P2, . . . , PNT, respectively, the transmission power adjusted transmission information can be represented as Formula 3.ŝ=[ŝ1,ŝ2, . . . ,ŝNT]T=[P1s1,P2s2, . . . ,PNTsNT]T  [Formula 3]
And, Ŝ may be represented as Formula 4 using a diagonal matrix P of the transmission power.
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        T                                                                                                        ]                                =          Ps                                    [                  Formula          ⁢                                          ⁢          4                ]            
Let us consider a case of configuring NT transmitted signals x1, x2, . . . , xNT, which are actually transmitted, by applying a weight matrix W to a transmission power adjusted information vector Ŝ. In this case, the weight matrix plays a role in properly distributing each transmission information to each antenna according to a transmission channel status and the like. The transmitted signals are set to may x1, x2, . . . , xNT may be represented as Formula 5 using a vector X. In this case, Wij means a weight between an ith transmitting antenna and a jth information. And, the W may be called a weight matrix or a precoding matrix.
                    x        =                                                       [                                                                                          x                      1                                                                                                                                  x                      2                                                                                                            ⋮                                                                                                              x                      i                                                                                                            ⋮                                                                                                              x                                              N                        T                                                                                                        ]                        =                                                            [                                                                                                              w                          11                                                                                                                      w                          12                                                                                            …                                                                                              w                                                      1                            ⁢                                                          N                              T                                                                                                                                                                                                                    w                          21                                                                                                                      w                          22                                                                                            …                                                                                              w                                                      2                            ⁢                                                          N                              T                                                                                                                                                                                          ⋮                                                                                                                                                                                          ⋱                                                                                                                                                       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T                                                                                                                                                              ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            w                                                                                    N                              T                                                        ⁢                            1                                                                                                                                                w                                                                                    N                              T                                                        ⁢                            2                                                                                                                      …                                                                                              w                                                                                    N                              T                                                        ⁢                                                                                                                  ⁢                                                          N                              T                                                                                                                                                            ]                                ⁡                                  [                                    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                                                                                                                              s                            ^                                                                                N                            T                                                                                                                                ]                                            =                                                W                  ⁢                                                                          ⁢                                      s                    ^                                                  =                WPs                                                                        [                  Formula          ⁢                                          ⁢          5                ]            
Generally, a physical meaning of a rank of a channel matrix may indicate a maximum number for carrying different informations on a granted channel. Since a rank of a channel matrix is defined as a minimum number of the numbers of independent rows or columns, a rank of a channel is not greater than the number of rows or columns. For example by formula, a rank of a channel H (i.e., rank (H)) is limited by Formula 6.rank(H)≤min(NT,NR)  [Formula 6]
Meanwhile, each different information sent by MIMO technique may be defined as ‘transport stream’ or ‘stream’ only. This ‘stream’ may be called a layer. If so, the number of transport streams is unable to be greater than a channel rank, which is the maximum number for sending different informations. Hence, the channel matrix H may be represented as Formula 7.#of streams≤rank(H)≤min(NT,NR)  [Formula 7]
In this case, ‘# of streams’ may indicate the number of streams. Meanwhile, it should be noted that one stream is transmittable via at least one antenna.
There may exist various methods for making at least one stream correspond to several antennas. This method may be described in accordance with a type of MIMO technique as follows. First of all, if one stream is transmitted via several antennas, it may be regarded as spatial diversity. If several streams are transmitted via several antennas, it may be regarded as spatial multiplexing. Of curse, such an intermediate type between spatial diversity and spatial multiplexing as a hybrid type of spatial diversity and spatial multiplexing may be possible.